A compressed air-driven screw tightening machine adapted to supply compressed air to an air motor (4) to rotatively drive a driver bit (9) and, concurrently therewith, to propel the driver bit (9) in the screw tightening direction, so as to the screw, wherein an exhaust path (24) for discharging into the atmosphere the compressed air discharged from the air motor (4) is provided with an exhaust control valve (30) for variably controlling the cross-sectional area of the exhaust path (24). The exhaust control valve (30) variably controls the amount of air being discharged from the air motor (4).